The requirements of a glass that is to be used in display technology, e.g., as a screen for a flat display, have been described in detail by W. H. Dumbaugh, P. L. Bocko and F. P. Fehlner ("Glasses for Flat-Panel Displays" in "High-Performance Glasses," edited by M. Cable and J. M. Parker, Blackie and Son Limited, Glasgow and London, 1992). The glasses that are available at this time for such uses are also listed in the article "Advanced Glass Substrates for Flat Panel Displays" by J. C. Lapp, P. L. Bocko and J. W. Nelson, Corning Research 1994.
Glasses that are to be used as substrates in LCDs (Liquid Crystal Displays), AMLCDs (Active Matrix Liquid Displays), TFELDs (Thin-Film Electroluminescent Displays) or PDPs (Plastic Display Panels) must have good resistance to thermal shock and good chemical stability with regard to reagents that are used in the production process of flat displays, and must be matched to the polycrystalline silicon as regards their thermal expansion behavior. In addition, these glasses should be as alkali-free as possible to keep integrated circuits, which can be placed directly on the glass substrate, from being "contaminated" by diffusing alkali ions. In this case, production-related proportions of sodium oxide can be tolerated up to a content of 1,000 ppm in the glass.
The glass panes that are suitable for the production of flat displays must have good dimensional stability at temperatures that occur in the production process, low shrinkage (compaction), and very good quality with regard to the absence of crystalline inclusions, knots, and bubbles.
Necessary properties of glasses for flat display uses are thus:
a thermal expansion coefficient .alpha..sub.20/300 of about 3.7.times.10.sup.-6 /K (for matching to polycrystalline silicon)
a viscosity of 10.sup.13 dPas at a temperature of above 700.degree. C. (for low compaction)
good chemical stability
low tendency toward devitrification.
The above-mentioned requirements are basically met by a commercially available glass (V1) which, according to an analysis, has approximately the following composition (in % by weight based on oxide): SiO.sub.2 57.7; B.sub.2 O.sub.3 8.4; Al.sub.2 O.sub.3 16.4: MgO 0.8; CaO 4.2; SrO 2.0; BaO 9.5; As.sub.2 O.sub.3 1.0. This glass is produced according to a special "overflow-fusion" process, which ensures the fabrication of thin glasses with high surface quality. The glasses that are suitable for this process, however, should exhibit only an extremely slight tendency toward crystallization, i.e., the liquidus temperature (any crystals that form dissolve again above this temperature) must lie considerably below the processing temperature V.sub.A (temperature for viscosity at 10.sup.4 dPas). This may also be the main reason for the very high V.sub.A value of this glass of &gt;1,300.degree. C. The higher V.sub.A is, however, the faster the corrosion of the refractory materials advances, and the higher the primary energy costs in the gas production from the mixture.
Another drawback of the above-mentioned glass VI is the high proportion of arsenic oxide, which obviously is necessary as a refining agent for ensuring the required freedom from bubbles with the selected production process.
Apart from the fact that owing to the toxicity of arsenic oxide it is prudent to avoid the use of these glass components, the presence of arsenic oxide leads to the fact that such glasses cannot be drawn on a float unit since the reductive conditions that are present here (liquid tin, reductive protective gas atmosphere, generally forming gas) lead to precipitation of metallic arsenic, which imparts an undesirable gray hue to the glasses, making them unusable.
In U.S. Pat. No. 3,496,401, glass composition ranges (% by weight based on oxide) that are similar to the composition of glass V1 are indicated for halogen light bulbs: SiO.sub.2 55-70; B.sub.2 O.sub.3 0-10 ; Al.sub.2 O.sub.3 13-25 ; alkaline-earth oxides 10-25 . No indications are given regarding refining, however. The thermal expansion values of these glasses are low; the temperatures for the viscosity of 10.sup.13 dPas are high. Their chemical stability is not described.
Japanese laid-open specification J 2-133 334 A describes alkali-free glasses for electronic components, which have good thermal stability, chemical stability, and optical homogeneity and have the following composition ranges (% by weight based on oxide): SiO.sub.2 54-60; B.sub.2 O.sub.3 6-10; Al.sub.2 O.sub.3 10-15 ; MgO 0-2 ; CaO 8-15 ; BaO 4-10 ; ZnO 1-6 ; TiO.sub.2 and/or ZrO.sub.2 0.3-4.
European laid-open specification EP 0 672 629 A2 describes aluminosilicate glasses for flat displays. It shows various composition ranges with various thermal expansion coefficients and different qualities relative to acid stability, which all require relatively large amounts of alkaline-earth oxides.
German laid-open specification DE-AS 20 58 210 describes borosilicate glasses that have separate phases and contain at least 55-70% by weight of SiO.sub.2, 1-8.3% by weight of B.sub.2 O.sub.3, 7-23% by weight of Al.sub.2 O.sub.3, and 6.7-16.5% by weight of MgO. These components ensure phase separation using heat treatment.